Method of reshaping metal matrix composite material

ABSTRACT

A method of reshaping high strength fiber reinforced metal composite sheets is disclosed. The sheets, which typically comprise unidirectionally oriented boron fibers in an aluminum matrix are metallurgically bonded to a pair of thin ductile metal face sheets. The resulting composite is then reshaped as desired by bending the composite along lines parallel to the fiber direction. Finally, the face sheets are removed, such as by acid etching, except in those areas in which it is desired to retain portions of the overlayer. Among other advantages, this technique permits much sharper radius bends by much simpler techniques that those previously used with such composite materials.

iinited States Patent 3,793,7o0

Maikish et al. Feb. 26, 1974 [5 METHOD OF RESHAPING METAL MATRIX3,667,108 6/1972 Schmidt 29/480 COMPOSITE MATERIAL 75 Inventors: CharlesR. Maikish; Herman R. Lazarus Attorney, Agent, or FirmJohn R. Duncan;Hugo F.

Wiant, both of Cajon, Calif. Mohflock [73] Assignee: General DynamicsCorporation, San

Diego, Calif. [57] ABSTRACT [22 Filed; S 1, 1972 A method of reshapinghigh strength fiber reinforced metal composite sheets is disclosed. Thesheets, which [21] Appl' 285684 typically comprise unidirectionallyoriented boron fibers in an aluminum matrix are metallurgically 52 us.Cl 29/419, 29/480, 29/481, bonded to a P of thin ductile metal facesheets- The 29/475 72/47 resulting composite is then reshaped as desiredby [51] Int. Cl B23p 17/00 bending the p it a ng lines p rall l t t fi r[58] Field of Search 29/480, 481, 4723, 475, 419 direction. Finally, theface sheets are removed, such as 29/424; 72/47 by acid etching, exceptin those areas in which it is desired to retain portions of theoverlayer. Among other [56] Ref Cit d advantages, this technique permitsmuch sharper ra- UNITED STATES PATENTS dius llaends (by r rzruch simplertechniques thin those pre- 3,378,916 4/1968 Robinson 29/419 x y use MSuc composlte mate 3,606,667 9/1971 Kreider 29/423 6 Claims, 5 DrawingFigures FABRlCATE FIBER/METAL MATRIX i COMPOSITE SHEET FORM DUCTILEMETAL N12 SURFACE kAYERS ON SHEET RESHAPE ASSEMBLY REMOVE PORTIONS 0F 16SURFACE LAYERS METHOD OF RESHAPING METAL MATRIX COMPOSITE MATERIALBACKGROUND OF THE INVENTION Recently, much interest has developed inhigh strength, light weight structural shapes made from compositematerials which utilize very high strength fibers or filaments embeddedin a metal matrix.

Typical composite materials comprise a plurality of substantiallyparallel boron fibers in an aluminum matrix. These materials have veryhigh strength properties in the fiber direction combined with relativelylow weight. These properties are especially desirable in a number ofaerospace applications.

Unfortunately, these materials are very difficult to reshape byconventional metalworking processes. Bending composite sheet materialsalong a line perpendicular to the fibers is very difficult because thefibers are so stiff they fracture rather than bend. Bending thecomposite along lines parallel to the fibers is also difficult since thematerial has a low transverse strength and the metal matrix tends torupture at the bend. I

Many structural shapes, such as columns, channels, T, and hat sections,effectively utilize the great unidirectional strength of compositematerials. Forming these shapes generally requires narrow-radius.

bends parallel to the fiber direction. While several techniques formaking these bends have been developed, none is entirely satisfactory.Preform sheets have been made with fiber density lower in the bendregion so that the composite is more ductile in these areas. However,this reduces sheet strength in those areas and requires careful handlay-up of the preform sheet. Generally, only sheets 0.03 inch thick andthinner can be formed without fiber damage and matrix cracking, and thenonly when hot-formed to radii of 6 times sheet thickness and larger.

Since the composite materials are relatively stiff and springy, it isdifficult to obtain accurate bends in one step. To eliminate springbackand obtain accurate size and shape, the sheet must be first bent to theapproximate final shape, then hot sized.

In fabricating structures from such composite materials problems areoften encountered in bonding the composite sheets to other structures.Direct bonding, such as by soldering, brazing or welding, of thealuminum composite matrix to other materials, such as steel or titaniumis often difficult. Similarly, fastening composite shapes to otherstructures by means of bolts or rivets is difficult due to the lowstrength of composites transverse to the fiber direction. Splitting ofthe composite material at rivet holes is common.

Thus, there is a continuing need for improved methods of fabricatingstructural shapes from composite materials made up of high strengthfibers in a metal matrix.

SUMMARY OF THE INVENTION It is, therefore, an object of this inventionto provide a process for reshaping metal matrix composite materialswhich overcomes the above noted problems.

Another object of this invention is to provide a method for reshapingmetal matrix composite materials useful at both room and elevatedtemperatures.

Still another object of this invention is to provide a technique forforming high quality small radius bends in metal matrix composite sheetmaterials.

Yet another object of this invention is to provide a method ofselectively reinforcing areas on formed metal matrix compositematerials.

The above objects, and others, are accomplished in accordance with thisinvention by a process for reshap- 1 ing composite sheets in which highstrength fibers are substantially unidirectionally embedded in a metalmatrix, which comprises forming a homogenous layer of ductile metal oneach surface of the composite sheet, bending the assembly as desiredalong lines which are substantially parallel to the fibers, thenremoving the surface metal layers from desired areas.

BRIEF DESCRIPTION OF THE DRAWING Details of the invention will befurther understood upon reference to the drawing wherein:

FIG. 1 is a flow sheet illustrating the steps in the process of thisinvention;

FIG. 2 is a schematic cross-section through a composite sheetcorresponding to the first step of FIG. 1;

DETAILED DESCRIPTION OF THE DRAWING Referring now to FIG. 1, there isseen a flow sheet for the process of this invention. Basically, theprocess begins with a first step, as indicated in box 10, of fabricatinga sheet of composite material comprising high strength fibersunidirectionally oriented in a metal matrix.

Any suitable high strength fibers may be used. Typical fibers includeboron fibers formed by depositing boron onto a very thin tungsten wire,boron fibers having a thin surface coating of silicon carbide (availablefrom Hamilton-Standard under the trademark Borsic"), silicon carbidefibers, nitride coated high modulus fibers and mixtures thereof. Typicalfibers have diameters from about 0.0040 to 0.0057 inch, tensile strengthgreater than about 400,000 psi and tensile modulus of elasticity of atleast about 55 X 10 psi. While any suitable fiber arrangement may beused, a parallel closely-spaced arrangement is preferred with about to200 fibers per inch width and with fiber content in the composite sheetof about 50 percent by weight. In general boron fibers are preferredbecause of their excellent physical characteristics.

Any suitable metal may be used for the fiberembedding matrix. Typicalmetals include aluminum, titanium, copper and alloys thereof. In generalbest results, with the optimum combination of high strength, goodformability and low weight are obtained with aluminum as the matrixmaterial. Typical preferred alloys include 2024 and 6061 aluminum.

The composite fiber-metal matrix sheets may be prepared by any suitableprocess. Typically, a metal foil may be placed around a large drum,fibers may be wrapped in a contiguous substantially parallel array, thenan adhesive may be sprayed over the fibers or a layer of aluminum may beplasma sprayed over the fibers to bond them to the foil. A plurality ofthese fiberfoil sheets, with or without interleaved sheets of foil, maybe stacked and bonded together, such as by diffusion bonding, under heatand pressure.

The end result of the first step, as indicated in box of FIG. 1, is acomposite sheet having a plurality of parallel high strength fibers in ametal matrix. A crosssectional view through this composite takenperpendicular to the fibers is shown in FIG. 2. Preform com prises ametal matrix 22 in which fibers 24 are embedded. Preform 20 may have anysuitable dimensions. Generally, best results are obtained where preform20 has a thickness of about 0.020 to 0.10 inch.

The second step of the process of this invention, as

illustrated in box 12 of the FIG. 1 flow sheet, comprises formingsurface layers of a ductile metal on each surface of preform 20, atleast in those areas which are to be bent or otherwise deformed. Theresulting assembly is illustrated in section in FIG. 3. If desired, anumber of composite fiber-containing sheets can be stacked with aductile metal sheet at the top and bottom, and the entire assembly canbe bonded together in one step.

Any suitable metal may be used in surface layers 32. Typical metalsinclude mild steel, stainless steel, nickel, titanium and alloysthereof. Ductile low carbon steel is generally preferred, since itproduces excellent bends when used in this process and may be easilyremoved without damage to the composite. Steel is also especiallyadvantageous when selected areas are not to be removed, but are leftbonded to the composite as reinforcements, fastening and bonding pads,etc.

The ductile metal surface layer may have any suitable thickness. Thethickness of layers 32 bears some relationship to the materials used andto the thickness of composite sheet 20. Generally, layers 32 should bethicker where formed from softer, more ductile metals or alloys. Also,thinner surface layers 32 can generally be used with thinner compositesheets 20. Typically, mild steel surface layers used on 0.02 to 0.05inch thick composite sheets comprising boron fibers in an aluminummatrix may have thicknesses ranging from about 0.015 to 0.025 inch.

Surface layers 32 may be formed by any suitable method. Typical methodsinclude metallurigical bonding of preformed sheets, such as by diffusionbonding; electroplating electroless plating, brazing, welding, etc.Metallurgical bonding of preformed sheets is preferred since it can bedone rapidly and economically.

Once surface layers 32 are applied, the assembly 30 is reshaped asindicated in the third step illustrated in FIG. 1 (box 14). A typicalhat section is shown in FIG. 4, produced by four .bending operations.Any shape which primarily requires bending around'lines parallel to thefiber orientation, such as angles, channels, Z sections, etc., may beconveniently formed by this process. Much tighter radius bends may beformed by this process than is possible without surface layers 32.In thepast, even with heating the composite to 800F, crackling and wrinklingof the composite sheet generally occurred if bends tighter than seventime the composite sheet thickness were attempted. With the presentprocess, bends as tight as three times the sheet thickness can beobtained at room temperature with composite sheets of about 0.030 inchthickness. While wider bends may be necessary with thicker compositesheets, the bend radius can be improved by heating the material. In anyevent, much narrower radius bends can be obtained by this process undera given set of conditions than was possible without surface layers 32.Furthermore, normal sheet metal working tools may generally be used toform the assembly 30, since it tends to bend along lines parallel to thefiber orientation much like a similarly dimensional sheet of mild steel.Without surface layers 32, composite sheets required complex tooling andcareful handling to obtain acceptable bends. The assembly 30 also hasless springback than does the basic composite sheet 20, so that bends toa final selected angle can be accomplished in one step, instead of thepreviously required separate rough forming and final sizing steps.

While surface layers 32 may be applied to composite 20 only in the bendregions, generally for simplicity and uniformity it is preferred tocompletely cover composite 20 with the surface layers.

The final step in this process, as illustrated in box 16 of FIG. 1, isthe removal of surface layers 32, at least from selected areas. FIG. 5illustrates a hat section from which surface layers 32 have been removedexcept from areas 40,42, and 44.

As seen in FIG. 5, portions of surface layer 32 may be left in place ifdesired. For example, if a rivet or other fastener is to be secured tothe composite, circular areas 40 and 42 may be left. These areas willthen serve as bearing surfaces and reinforcements when a hole 44 isdrilled out for the fastener. Similarly, a bonding pad 46 may be leftwhere it is desired to secure a member 48 to the composite structure.Thus, where the metal matrix is aluminum, surface layer 32 and pad 46may be mild steel, so that sheet 48, which may be steel, can be moreeasily bonded by conventional brazing or welding. Also, since compositematerials have highly unidirectional strength characteristics, portionsof layer 32 may be left in selectedareas toincrease the strength ofthose areastransverse to the fiber direction.

The portions of surface layers 32 to be removed may be removed by anysuitable process. Typically, surface layers 32 may be dissolved by asolvent or etching solution which does not attack the composite, or theymay be mechanically removed by grinding or machining. Chemical etchingis preferred, since it is rapid, efficient, and does not damage thecomposite. Areas to be left, such as areas 40, 42 and 46 in FIG. 5, maybe retained by merely coating them with a material resistant to theetchant, then removing the resist coating after etching is complete.

The following examples describe in detail several preferred embodimentsof the process of this invention. Parts and percentages are by weightunless otherwise indicated.

EXAMPLE 1 Two packs, each comprising six layers of boronlaluminumsheets, available from the Hamilton- Standard Division of UnitedAircraft, are prepared. Each sheet consists of an aluminum foil having asurface layer of parallel contiguour Borsic (silicon carbide coatedboron) fibers bonded to the foil by plasmasprayed aluminum. Each sheethas an uncompacted thickness of about 0.009 inch.

The packs are assembled by stacking the sheets with all fibersunidirectionally oriented and two outer layers of 0.0014 inch 6061aluminum foil. The outer surfaces of the first pack are coated withNicrobraze-red, a stopbond material, and two steel caul plates areplaced above and below the pack. The outer surfaces of the second packare wire brushed and thoroughly cleaned with acetone. Two 0.014 inchfully annealed mild steel sheets are also cleaned and placed above andbelow the second pack. Each pack is then diffusion bonded in a highpressure autoclave at about 10,000 psi and 900F for about 60 minutes.Pressure is then lowered to about 3,500 psi, the temperature is raisedto about 975F. After about minutes, the autoclave is cooled anddepressurized. The first bonded composite sheet has a compactedthickness of about 0.033 inch and the second bonded composite sheetabout 0.061 inch (due to the bonded steel surface plates). Each sheet isthen bent on conventional production metal working equipment into hatsections such as is shown in FIGS. 4 and 5 of the drawing. Bend radiusis about three times thickness, or about one-eighth inch. The secondcomposite sheet is then immersed in a 50 percent solution of nitricacid. The mild steel surface layers are rapidly etched away withoutdamage to the boron/aluminum composite. Visual and radiographicexamination of the second composite sheet shows bends of excellentquality, with substantially no cracking of the aluminum matrix or damageto the boron fibers. Examination of the first composite sheet showssevere cracking of the aluminum matrix at the bends with fracturing anddistortion of fibers near the surface.

Example 11 Ten boron/aluminum sheets, each consisting of a singlecollinated contiguous layer of boron fibers diffusion bonded between two6061 aluminum foils are stacked with the fibers unidirectionallyoriented. Each sheethas an uncompacted thickness of about 0.007 inch.The outer surfaces of the stack are carefully cleaned and two sheets of0.2 carefully cleaned mild steel are placed thereagainst. The resultingpack is placed in a high pressure autoclave and diffusion bonded at10,000 psi and 900F for about three hours. After cooling andprepressurization, the pack is removed from the autoclave. The resultingcomposite plate has a compacted thickness of about 0.110 inch. Usingconventional sheet metal forming tooling such as would be used to bend aplain steel plate of equal thickness, the composite plate is bentparallel to the fiber direction to a radius of about 0.2 inch. Selectedareas on the plate are then treated with Turcoform Maskant 538 (TurcoCorp.), an acid resistant material. Typically, circular areas onopposite sides of the sheet are treated, as is a generally rectangulararea on one surface. The plate is then placed in a 50 percent nitricacid solution to etch away the steel surfaces except where protected bythe coat- Example lll Six sheets each consisting of a parallelcontiguous array of Borsic fibers bonded to the surface of an aluminumfoil are stacked with fiber-bearing surfaces uppermost. A 0.014 inchaluminum foil is added to the stack, then six more Borsic fiber/aluminumfoil sheets are added with the fiber-bearing surfaces'downward. Finally,two 0.01 titanium sheets-are placed over the outside of the stack. Theresulting composite pack is coated with Nicrobraze-red stop-offmaterial, placed between caul plates. The pack is heated to about 950Fand maintained under about 10,000 psi pressure for about 2 hours. Afterpressure is removed and the re- 1 sulting compacted composite plate iscooled to room temperature the stop-off material is removed and theouter surfaces of the plate are thoroughly cleaned. The composite plateis then overcoated with a 0.01 inch copper layer by conventionalelectroplating techniques. The assembly is then bent along linesparallel to the fiber direction by conventional sheet metal bendingequipment to form a Z section. The copper surface layers are removed bynitric acid etching. The resulting composite shape has bends ofexcellent quality, with no evidence of matrix fiber damage.

While specific conditions, proportions and arrangement have beendescribed in the above examples of preferred embodiments, these may bevaried, as discussed above, with similar results. Other modifications,applications and ramifications of the invention will become apparent tothose skilled in the art upon reading this disclosure. These areintended to be included within the scope of this invention as defined inthe appended claims.

We claim:

1. A method of shaping high strength fiber reinforced metal compositesheets which comprises:

providing a composite sheet in which high strength fibers areunidirectionally arrayed in a metal matrix,

metallurigically bonding a sheet of ductile metal having substantiallyuniform strength characteristics in all directions to each major surfaceof said composite sheet to form a sandwich structure,

bending said sheet as desired about lines which are substantiallyparallel to said fiber direction; and removing at least portions of saidductile metal sheets from said composite sheet.

2. The method according to claim 1 wherein said composite sheetcomprises high strength fibers comprising boron embedded in an aluminummatrix.

3. The method according to claim 2 wherein said ductile metal sheetscomprise mild steel sheets diffusion bonded to said composite sheet.

4. The method according to claim 3 wherein said ductile metal sheets areat least partially removed by treating selected areas with nitric acid.

5. The method according to claim 4 where said composite sheet has athickness of from about 0.02 to 0.05 inch, said ductile metal sheetshave thicknesses of from about 0.015 to 0.025 inch and said sheet isbent to a ra- 7 8 dius as narrow as three times the thickness of thecomto each face of said composite sheet to form a uniposite sheet. tarysandwich structure;

6. A method of reshaping high strength fiber reinreshaping said sheet bybending said sandwich as deforced metal composite sheets whichcomprises: sired about lines which are substantially parallel toproviding a composite sheet comprising high strength said fibers; and

boron fibers substantially unidirectionally arrayed treating at leastportions of said sandwich with nitric in an aluminum matrix; acid todissolve away the treated portions.

metallurigically bonding a thin mild steel cover sheet

2. The method according to claim 1 wherein said composite sheetcomprises high strength fibers comprising boron embedded in an aluminummatrix.
 3. The method according to claim 2 wherein said ductile metalsheets comprise mild steel sheets diffusion bonded to said compositesheet.
 4. The method according to claim 3 wherein said ductile metalsheets are at least partially removed by treating selected areas withnitric acid.
 5. The method according to claim 4 where said compositesheet has a thickness of from about 0.02 to 0.05 inch, said ductilemetal sheets have thicknesses of from about 0.015 to 0.025 inch and saidsheet is bent to a radius as narrow as three times the thickness of thecomposite sheet.
 6. A method of reshaping high strength fiber reinforcedmetal composite sheets which comprises: providing a composite sheetcomprising high strength boron fibers substantially unidirectionallyarrayed in an aluminum matrix; metallurigically bonding a thin mildsteel cover sheet to each face of said composite sheet to form a unitarysandwich structure; reshaping said sheet by bending said sandwich asdesired about lines which are substantially parallel to said fibers; andtreating at least portions of said sandwich with nitric acid to dissolveaway the treated portions.